Hydrogenated amorphous silicon, hereinafter sometimes abbreviated as a-Si:H, is a useful semiconductor material which has applications, for example, in the formation of thin film solar arrays, photoreceptors for electrophotography, image sensors for facsimile transmission, fast detectors, switching diodes, and the like. Also, thin film alloys of hydrogenated amorphous silicon with other elements, such as carbon, germanium, nitrogen, fluorine, etc., are used in tandem solar cells. However, the methodology for producing films of hydrogenated amorphous silicon, and for producing the gaseous precursor materials, disilane and higher silanes (collectively hereinafter sometimes referred to as polysilanes), is rather costly when applied on an industrial scale.
It would therefore be desirable to obtain a low-cost, efficient method for forming polysilanes and polygermanes and for the deposition of hydrogenated amorphous silicon, alloys thereof, or hydrogenated amorphous germanium.
The present invention provides an advantage over the prior art methods of forming polysilanes, polygermanes, specifically for forming disilane, and forming films of hydrogenated amorphous silicon, alloys thereof, or hydrogenated amorphous germanium. A commonly used method in the prior art to form hydrogenated amorphous silicon films is by the decomposition of monosilane, SiH.sub.4, on the surface of a heated substrate in a DC or RF glow discharge. The glow discharge technique suffers the major disadvantage of extremely low deposition rates, usually on the order of a few angstroms per second. Furthermore, this method suffers from the disadvantage of continuous bombardment of high energy charged particles on the growing a-Si:H surface, thereby introducing defects into the film.
Higher deposition rates may be obtained by using the method of chemical vapor deposition (CVD) through the thermal decomposition of disilane or higher silanes. This method can achieve deposition rates of 100 angstroms per second or higher. Although the CVD method is easier to scale up, and produce better quality films than the glow discharge method due to the absence of charged particles, the CVD method is dependent on the use of high-purity disilane and higher silanes. These precursor gases have not heretofore been available at low cost. In particular, the diluent (or carrier gas) for disilane, such as hydrogen or helium, must be thoroughly purified by gettering and other techniques to minimize contamination in deposited films.
In the present invention high-purity disilane and higher silanes or germanes are produced which allows for the production of high quality hydrogenated amorphous silicon alloys thereof or hydrogenated amorphous germanium, by the CVD method.
The present invention provides further advantages in the aspect of the deposition of hydrogenated amorphous silicon, alloys thereof, or germanium. One of the deposition methods used in the prior art is a static system. The deposited films from static systems are not usually homogeneous in the thickness direction due to variation in the composition of the reaction mixture with time. Furthermore, by use of the static method, residual impurities in the deposition apparatus may be incorporated into the deposited films.
In another method used in the prior art, the low pressure CVD process, the purity of the deposition equipment is extremely critical since any traces of contaminants are readily incorporated into the deposited films. Low pressures (2 to 30 torr) of reactants are used by this method.
Deposition under atmospheric pressure in a gas flow system has been utilized as shown, for example, in U.S. Pat. No. 4,439,463. However, a major disadvantage of the approach is that the precursor disilane and/or higher silanes are first produced by electrical discharge in a separate reaction chamber at low pressures. This necessitates an apparatus which requires maintenance of a pressure differential between the chamber in which the disilane and higher silanes are formed (at low pressures) and the chamber in which the deposition occurs (at atmospheric pressure). This unnecessarily complicates the equipment which must be utilized, and therefore the capital expense of carrying out this process on an industrial scale.
The present invention provides improved deposition methods whereby uniform, high-quality a-Si:H films, alloys thereof, or a-Ge:H may be deposited rapidly and efficiently using a low-cost apparatus.